In a comparative study of Gas-to-Liquids (GTL) diesel and conventional Ultra Low-Sulfur Diesel (ULSD) (and blends), researchers at the University of Birmingham (UK) and Brunel University (UK) have found that the optimized use of GTL and GTL blends can reduce NOx and PM emissions simultaneously while providing improvements in engine thermal efficiency.

In a paper just published in Energy & Fuels, the team also describes their finding that the use of GTL with exhaust-gas reforming—passing the exhaust gas through a minireformer installed at the EGR to generate hydrogen for injection into the fuel charge—increased fuel conversion to hydrogen and reduced methane production.

The researchers used a single-cylinder direct-injection diesel engine with a pump-line-nozzle-type fuel injection system. (Results may vary for engines equipped with common rail or unit injection systems.) The fuels used were ULSD and GTL provided by Shell Global Solutions, UK, and a 50-50 blend of GTL and ULSD.

The team found that use of GTL with unoptimized injection timing resulted in a significant reduction of NOx, but with an increase in smoke emissions due to shorter premixed combustion and longer diffusion combustion—especially at high loads.

Use of the 50-50 blend in the same unoptimized engine resulted in reduction in smoke without significant changes in the NOx values. Both neat GTL and the blend showed enhanced fuel economy on a gravimetric basis and better engine thermal efficiency.

However, by advancing the injection by 4° crank angle and allowing the combustion of GTL to shift to earlier stages, they found that both NOx and smoke emissions decreased simultaneously, while improving engine thermal efficiency compared to the ULSD.

The reduction of the premixed combustion phase in the case of GTL fuel allows injection timing to be advanced, which results in improved engine efficiency while still maintaining NOx and combustion noise at low levels. In the case of ULSD, advancing the injection timing can lead to improved engine thermal efficiency but this will result in the considerable increase of NOx emissions and engine combustion noise. At the lowest load condition...the results suggest that the same engine power could be generated by 10% less fuel energy when GTL is employed. This benefit of GTL has been overlooked in a recent influential well-to-wheels study [CONCAWE, earlier post].

Major components of the REGR (reforming and exhaust-gas recirculation) system. Click to enlarge.

The use of the REGR (reforming and exhaust-gas recirculation) system, which puts a small minireformer inline with the EGR path, is part of a separate, ongoing study. The reformer is a largely passive, adiabatic reactor, and must be able to function under variable feed rates and inlet temperatures with minimal external control.

In using the REGR, the researchers found that GTL exhaust delivered more hydrogen with greater process efficiency than the ULSD. As a further benefit in such an application, GTL’s sulfur- and aromatic-free properties can also increase the long-term performance and durability of the catalyst.

The fuel properties (e.g., density, viscosity, CN, aromatic HC) and hence combustion of the new fuels such as GTL and biofuels are different from conventional fuels, and this will affect the engine performance and emissions. Similarly, the catalyst and reformer design have to be optimized according to the specifications of the new fuels in order to value the beneficial effects.

For this reason, and those related to the production of small volumes, these fuels will be likely to be used as blends with conventional fuels. Although, modern diesel engine injection systems such as common rail (CR) are less sensitive to fuel properties (e.g., CN) compared to pump-line-nozzle injection systems, engine tuning will be required in order to accomplish the highest improvements in engine performance.

in Europe, both BP Ultimate and Shell V-Power premium diesel grades contain a small percentage of GTL already. A whole slew of tests must be successfully completed before such a fuel composition change can be implemented. The following article from 2005 provides more detail on this process and the results:

Bottom line: xTL liquids are superior to dino diesel because of their extremely high cetane number. This permits sharp reduction in engine out concentrations of unburnt products (CO, HC and PM), which implies slightly higher thermodynamic efficiency and slightly better fuel economy. NOx levels were a few percent higher, consistent with the higher temperatures associated with more complete combustion. Note that xTL liquids are free of sulphur (it would foul the F-T catalysts), so the particles produced contain no sulphuric acid (cp. biodiesel).

The study described above used ULSD as a baseline. The benefits are more pronounced because the cetane number differential is larger than in Europe. The NOx reduction is a little puzzling. Perhaps the combustion chamber geometry and/or fuel injection system plus control strategy were suboptimal for regular diesel to begin with.

Conclusion: synthetics are better than dino diesel but not yet by enough to justify the higher cost (~2x for CTL/GTL, ~3x for BTL). Producers are keen to push xTL because it lets them add a lot of value to (domestic) coal and stranded gas deposits (Malaysia, Qatar), thus topping up the proven reserves on their balances sheets.

The primary relevant advantage of xTL liquids are reduced engine-out PM emissions, since CO and HC can be cleaned up with oxycats and dino diesel is now ultra-low in sulphur, too. Note that even if you have aftertreatment systems installed, you want to keep engine-out PM and NOx levels as low as possible to ensure system longevity and, to minimize the impact on fuel economy.

GTL is no good in comparison to dual fuel diesel (45% ish and natural gas 55% ish). Bring the natural gas to UK from Qatar as LNG, only uses around 8 - 10% of the fuel from the gas reservoir to the vehicle in the UK (stored as LNG). No complex plant (LNG simple).....

Then, when you burn this mix you get 20% less CO2 than you would from diesel alone (due to lower carbon content of methane). far lower NOX as well.

To make it even better, use some bio-diesel in the diesel and bio-methane in the natural gas - total about 20 - 50% lower CO2 than GTL!! And a lot cheaper as making GTL is incredibly capex and energy intensive.

DME, chemical formula H2-C-O-C-H2, emits zero soot at combustion because of low carbon chain length, further divided by oxygen atom. It is by far the cleanest burning hydrocarbon capable to be directly used in compression ignition (“diesel”) engines.

Being gas at normal temperature and atmospheric pressure, it is liquid at moderate pressure and as such to diesel engines it is like propane to gasoline. But again, it has high cetane number and could be used directly in diesel engines.

Very interesting possibility is to use DME as dual fuel in diesel engine. Portion of DME could be evaporated on engine coolant heat exchanger and perfectly mixed with intake air to produce mixture lower self-ignition level in particular diesel engine, but higher then lower combustion limit. Exactly this practice is used in dual fuel NG and LPG engines. Small part of DME could be injected as liquid by traditional diesel fuel injectors near TDC and initiate ignition and combustion of whole charge. Since most of fuel is homogenously mixed and highly diluted below stoichiometric level with air, combustion is complete and very lean, which translates in near zero generation of NOx and PM. Combustion of liquid injected portion of DME still produces NOx, but engine could be tuned to keep it ratio very low – about 5-10% from all calorific value of fuel consumed. Thus DME diesel could utilize all advantages of dual-fuel engine – but on one fuel.

Additional benefits of DME is absence of evaporative emission during refueling, and total elimination of water/soil contamination if spill occurs. No sea contamination will occur if tanker carrying DME will sink. Disadvantage is the same as with propane: potential explosion hazard if DME leaks in closed environment, like closed garage, but explosion hazard is way lower compared to high-pressure fuels like CNG or H2.
Fire hazard is substantially lower then on gasoline engine.

dual fuel implies maintaining two fuel tanks. This is currently still a non-starter for the vast majority of applications.

Natural gas has a high octane number (RON 130 for pure methane) but even that will not permit compression ratios much above 14 if you're using port fuel injection. Since NG takes up more volume than liquid fuel does, you have to increase the boost pressure to maintain specific power - i.e. your tubo lag goes up.

DME has a high cetane number, you cannot use port fuel injection for it. On the other hand, it's liquid at mildly elevated pressures. Therefore, a diesel-DME mix could be used provided your distribution infrastructure, fuel dispensers and in-vehicle fuel tanks remained pressurized at all times. The downsides are very high cost of this fuel system plus DME as such and, DME's relatively low energy density.

I wish that I could convert my "96 VW Passat to dual fuel capability. Even a small tank that I had to refuel daily would be satisfactory for my driving needs. Fewer oil changes, encreased engine life, but most importantly less pollutuion and the dollars would circulate in my local community.